Clustering of cell bodies, bundling of dendrites, and gap junctions: morphological substrate for electrical coupling in the prepacemaker nucleus

The possible morphological basis for electrical coupling between neurons of the prepacemaker nucleus was studied in weakly electric gymnotiform fish at the ultrastructural level. Three structural characteristics were found: Extremely dense clustering of cell bodies; 'bundling' of dendrites; and gap junctions between neurons. Electrical coupling may take place through gap junctions and the spatial arrangement of elements in the prepacemaker nucleus, which could enable ephaptic interactions. Such mechanisms may also be used for averaging the responses of individual neurons in the whole assembly in order to render more predictable behavioral reactions.